1. Field of the Invention
This invention relates to a method of preparing a magnetic coating material. This invention particularly relates to a method of preparing a kneaded mixture for a magnetic coating material by use of a two-shaft continuous kneading and mixing machine.
2. Description of the Prior Art
In the production of a magnetic recording medium, in order to quickly obtain a magnetic coating material having a high dispersing quality, there has heretofore been known to carry out a series of processes for kneading ferromagnetic grains and a small amount of a binder solution by use of an incorporating machine such as a kneader exhibiting high shearing force before the ferromagnetic grains are dispersed in the binder solution by use of a dispersing machine such as a ball mill or a sand grinder. Also, in order to obtain a uniform paste by the kneading, it has heretofore been known to increase the kneading effect by use of a dispersant and/or a stabilizer or by use of a binder within a specified range of concentration. The techniques with respect to these preparation methods are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 46(1971)-3030, 48(1973)-104505, 49(1974)-14537, 53(1978)-76012, 55(1980)-25406, and 57(1982)-141026.
Techniques for kneading by use of a continuous kneading and mixing machine in the kneading process are described in, for example, "Kurimoto Giho", No. 15, Chapter 9, separate volume, (1986, 7), Kurimoto, Ltd., and Japanese Unexamined Patent Publication Nos. 62(1987)-36472 and 62(1987)-41274. However, nothing has heretofore been proposed concerning a method of kneading reliably with very high shearing force.
On the other hand, as one of the methods for kneading with shearing force higher than in the conventional techniques, it has heretofore been known to carry out the kneading by minimizing the amount of a solvent in the kneaded mixture. Specifically, with the known method, in order to exert high shearing force, it is necessary for the kneading to be carried out at as high a viscosity as possible. However, in the case where the kneading is carried out by use of the minimum amount of the solvent, the kneading may become impossible and cannot be carried out reliably depending on fluctuations in external factors such as the temperature and humidity, and fluctuations in raw materials, particularly fluctuations in physical properties, for example, the water content, bulk density and specific surface area of the ferromagnetic grains.
The same problems arise in the case where the kneading is carried out by use of the two-shaft continuous kneading and mixing machine. Specifically, at the time the kneading is to be started by use of the two-shaft continuous kneading and mixing machine, the kneading vessel has not been loaded with a composition for the magnetic coating material to be kneaded, and thus is empty. In the case where a high-viscosity composition for the magnetic coating material is introduced into the kneading vessel in this condition, no back pressure is given rise to in the two-shaft continuous kneading and mixing machine, and idling readily arises. Also, the kneading may become impossible and cannot be carried out reliably depending on fluctuations in the external factors and fluctuations in raw materials.